Specify that the default value for `cs`, in the absence of a fallback in-scope `<map-meta>`, is `gcrs`

[prushforth]

When all signals have been exhausted in processing a MapML document, when interpreting <map-coordinates> content we have to have a default value that is specified by the MapML spec. Recent inspection fails to turn up such a statement, so this is a spec bug.

[prushforth]

It's also a documentation bug:

If no fallback coordinate system is identified by a element, the default value of pcrs (projected coordinates) is used.

[prushforth]

The map-feature code uses gcrs